1. Field of the Invention
The present invention relates to a motor vehicle position recognizing system for recognizing or detecting with high accuracy a position of a motor vehicle such as automobile, car, track or the like objects moving or traveling on a road on the basis of magnetic field intensities generated at predetermined positions on a road surface and detected by a magnetic field intensity detecting means mounted on the vehicle.
2. Description of Related Art
In recent years, there have been proposed and developed for practical applications a variety of motor vehicle position recognizing systems in which magnetic field generating means are laid on a road for generating or forming predetermined magnetic fields at predetermined positions on the road surface for thereby detecting with high accuracy the vehicle position on the basis of the magnetic field intensity detected by the magnetic field intensity detecting means mounted on the motor vehicle traveling on the road surface with a view to making it possible to realize an automatic steering control of the motor vehicle and a lane departure alarming function.
By way of example, the hitherto known or conventional motor vehicle position recognizing system disclosed in Japanese Patent Application Laid-Open Publication No. 206173/1998 (JP-A-10-206173) is comprised of magnetic field generating means or magnetic markers laid on the road surface at predetermined positions for generating or forming predetermined magnetic fields and a magnetic field intensity detecting means (magnetic sensor) mounted on a motor vehicle for detecting the magnetic field emanated from the magnetic markers, for thereby arithmetically determining the vehicle position on the basis of the magnetic field intensity detected by the magnetic sensor.
Further, the conventional motor vehicle position recognizing system mentioned above includes a vehicle position arithmetic means which incorporates therein a vehicle position arithmetic operation propriety decision means for making decision as to propriety of the arithmetic determination concerning the position of the motor vehicle so that the arithmetic operation for determining the motor vehicle position on the basis of the field intensity detected by the magnetic sensor is performed in dependence on the result of the decision made by the vehicle position arithmetic operation propriety decision means.
In addition, the conventional motor vehicle position recognizing system mentioned above includes a nonmagnetic field position determining means for determining nonmagnetic field position which is insusceptible to the influence of the magnetic sensor (e.g. mid or intermediate position between the magnetic markers) so that the magnetic field intensity detected by the magnetic sensor can be corrected by referencing the magnetic field intensities at the nonmagnetic field positions recorded in advance.
With the conventional motor vehicle position recognizing system described above, the magnetic field intensity can be determined without suffering the influence of the terrestrial magnetism and the magnetic field(s) emanated from the motor vehicle itself. Thus, the vehicle position can be detected or recognized with reasonable accuracy.
In this conjunction, it is however noted that the characteristics and the types of the magnetic markers may differ from one another. Besides, the height of the magnetic sensor mounted on the motor vehicle relative to the magnetic marker may undesirably change in dependence on the conditions of the road surface (e.g. roughness, unevenness, snowfall, etc.).
FIG. 7 is a block diagram showing generally and schematically a conventional motor vehicle position recognizing system such as disclosed, for example, in Japanese Patent Application Laid-Open Publication mentioned above.
Referring to FIG. 7, a magnetic field generating means implemented as a magnetic marker or markers 1 are laid on a road for the motor vehicles.
Parenthetically, only one magnetic marker 1 is representatively shown in the figure. It should however be appreciated that a plurality of the magnetic markers 1 may of course be laid on the road, for example, in the form of a marker row or rows.
In this conjunction, the magnetic markers 1 of a marker row laid at a center position on a lane may be arrayed such that the magnetic polarities of the magnetic markers change alternatively, e.g. in the order of N, S, N, S, . . . . On the other hand, in the marker row laid on the lane, being offset to the right from the center marker row by e.g. 1 m, the magnetic markers may be arrayed with a uniform polarity, e.g. in the sequence of N, N, N, N, . . . . Similarly, in the marker row laid with the offset to the left from the center marker row by e.g. 1 m, the magnetic markers may be arrayed with the opposite polarity, e.g. in the sequence of S, S, S, S . . . .
Installed on the motor vehicle running on the road surface on which the magnetic markers 1 are laid is a motor vehicle position recognizing apparatus 4 which is composed of magnetic sensors 2 and a vehicle-onboard detection unit 3. The magnetic sensors 2 are disposed at positions opposite to the magnetic marker rows, respectively. The output signals of the magnetic sensors 2 are supplied to the vehicle-onboard detection unit 3.
More specifically, a plurality of the vehicle-onboard magnetic sensors 2 are disposed in the transverse direction of the motor vehicle (i.e., in the direction orthogonal to the running or traveling direction), e.g. at right, center and left positions, respectively, as is illustrated in FIG. 7.
Further, the motor vehicle is equipped with various types of sensors inclusive of a vehicle speed sensor 5 and others, wherein detection signals outputted from these sensors are inputted to the vehicle-onboard detection unit 3 as well.
The vehicle-onboard detection unit 3 is designed to determine arithmetically a transverse position Phr of the relevant motor vehicle on the basis of the output signals of the magnetic sensors 2, the vehicle speed sensor 5 and others.
Next, by referring to FIGS. 8A, 8B and 8C, description will briefly be made of the operation of the conventional motor vehicle position recognizing system shown in FIG. 7.
FIGS. 8A, 8B and 8C are views for illustrating, respectively, the strength distribution of the output signal of the magnetic sensor 2 and intensities of a vertical magnetic field intensity signal (hereinafter also referred to as the vertical field intensity signal) and a horizontal magnetic field intensity signal (hereinafter also referred to as the horizontal field intensity signal) detected in correspondence to the position of the motor vehicle in the running or traveling direction in dependence on the positional relations between the magnetic sensors 2 and the magnetic markers 1 (i.e., positional deviation Ph of the motor vehicle in the transverse direction).
Each of the magnetic markers 1 laid on the road at predetermined positions generates or forms a magnetic field of a predetermined intensity on or above the road surface. Thus, the motor vehicle equipped with the motor vehicle position recognizing apparatus 4 moves through the magnetic fields mentioned above.
At that time, the plural onboard magnetic sensors 2 output the magnetic field intensity signals in response to the intensities of the magnetic fields generated by the magnetic markers 1, respectively.
The vehicle-onboard detection unit 3 fetches the magnetic field intensity signals from the outputs of the magnetic sensors 2 while sampling the magnetic field intensity signals in dependence on the engine operation state detection signals derived from the outputs of the vehicle speed sensor 5 and others.
The vehicle-onboard detection unit 3 is so designed as to detect the magnetic field intensity distribution in the magnetic field space to thereby determine arithmetically the position Phr of the motor vehicle in the transverse direction on the basis of the spatial field intensity distribution.
The signal outputted from the vehicle-onboard detection unit 3 and indicating the lateral or transverse position Phr of the motor vehicle determined arithmetically as mentioned above is then inputted to an electronic control unit (ECU) (not shown) incorporated in a motor vehicle control apparatus (not shown either) which is so designed as to issue an alarm message or perform a vehicle steering control in response to the input signal indicating the lateral or transverse position Phr of the motor vehicle.
By way of example, let""s assume that one of the magnetic sensors 2 installed on the motor vehicle spatially displaces from the left side to the right side as viewed in FIG. 8A with a positional deviation Ph (refer to double-arrows) in the lateral or transverse direction relative to the magnetic marker 1, as illustrated in FIG. 8A.
In that case, the magnetic sensor 2 outputs a vertical field intensity signal and a horizontal field intensity signal in response to the vertical field intensity and the horizontal field intensity, respectively, of the magnetic field generated by the magnetic marker 1, as can be seen in FIGS. 8B and 8C. These magnetic field intensity signals assume maximum or peak values Vv and Vh, respectively, at the position where the magnetic sensor 2 approaches most closely to the magnetic marker 1.
The vehicle-onboard detection unit 3 arithmetically determines the positional deviation Ph of the magnetic sensor 2 in the lateral or transverse direction relative to the magnetic marker 1 on the basis of the maximum or peak value Vv of the vertical field intensity signal and the maximum or peak value Vh of the horizontal field intensity signal as determined.
At this juncture, it is to be mentioned that the positive/negative polarity (i.e., plus/minus sign) of the individual magnetic field intensity signals and the peak values Vv and Vh are determined in dependence on the polarity of the magnetic marker 1.
Next, by reference to a flow chart shown in FIG. 9 together with FIG. 10, description will be made in the concrete of operation of the vehicle-onboard detection unit 3 of the conventional motor vehicle position recognizing system.
FIG. 10 is a view for graphically illustrating the peak values Vv and Vh of the magnetic field intensity of the magnetic sensor 2 of concern, installation height h at which the magnetic sensor 2 is mounted and the transverse positional deviation Ph of the magnetic sensor 2 relative to the magnetic marker 1 (i.e., positional deviation of the magnetic sensor in the transverse direction relative to the magnetic marker).
Referring to FIG. 9, the vehicle-onboard detection unit 3 arithmetically determines at first a vehicle speed (i.e., speed of the motor vehicle) on the basis of the output signal of the vehicle speed sensor 5 (step S1).
Subsequently, sampling time for sampling the output signal of the magnetic sensor 2 at a predetermined distance interval (e.g. per 5 cm) is set on the basis of the vehicle speed determined arithmetically in the step S1 (step S2).
In succession, the output signal of the magnetic sensor 2 is sampled at the sampling time interval set in the step S2 to be subsequently stored (step S3). In this manner, the magnetic field intensity signal derived from the output of the magnetic sensor 2 is stored in a storage unit (not shown) which is in corporated in the vehicle-onboard detection unit 3 to be used as the data concerning the spatial field intensity distributions.
Parenthetically, although it has been described above that the sampling time is so set in succession to the arithmetic determination of the vehicle speed that the sampling interval corresponds to the predetermined distance interval (steps S1 to S3). It should, however, be understood that the covered distance may directly be determined arithmetically on the basis of output pulses contained in the output signal of the vehicle speed sensor 5 to thereby effect the sampling of the magnetic field intensity signal every time when the covered distance reaches predetermined values, respectively.
Next, the maximum point, i.e., the peak value, is determined on the basis of the spatial field intensity distribution data acquired from the output of the magnetic sensor 2 in the step S3 to thereby determine arithmetically the peak value Vv of the vertical field intensity as well as the peak value Vh of the horizontal field intensity (step S4).
In succession, the magnetic sensor 2 which exhibits the greatest absolute value of the maximum vertical field intensity among the magnetic field intensity signals derived from the plural magnetic sensors 2 is selected to be subjected to the arithmetic processing, whereon the peak vertical field intensity value Vv and the peak horizontal field intensity value Vh derived from the output of the magnetic sensor 2 selected as the object for arithmetic processing are set as the ultimate peak values, respectively.
Subsequently, the positional deviation Ph of the magnetic sensor 2 in the transverse direction relative to the magnetic marker 1 is arithmetically determined in accordance with the undermentioned expression (1) on the basis of the ultimate peak values Vv and Vh as determined and the installation height h (refer to FIG. 10) of the magnetic sensor 2 selected as the object for the arithmetic processing (step S5):
Ph=kxc3x97hxc3x97Vh/Vvxe2x80x83xe2x80x83(1)
where k represents a predetermined coefficient.
In this conjunction, it is to be mentioned that although it has been described that the positional deviation Ph of the magnetic sensor 2 in the transverse direction relative to the magnetic marker 1 is arithmetically determined in accordance with the expression (1) mentioned above, it should, however, be understood that such positional deviation Ph of the magnetic sensor can equally be determined on the basis of map data determined empirically through experiments and stored in advance.
Subsequently, the vehicle-onboard detection unit 3 adds an offset quantity Os of the magnetic sensor 2 subjected to the arithmetic processing from the center position of the motor vehicle to the positional deviation Ph of the magnetic sensor 2 in the transverse direction to thereby determine arithmetically a positional deviation Phc of the center position of the motor vehicle in the transverse direction relative to the magnetic marker 1 in accordance with the following expression (2):
Phc=Ph+Osxe2x80x83xe2x80x83(2)
At this juncture, it should be recalled that the magnetic markers 1 laid on the road surface are implemented in the form of the marker row laid at the center of the lane and the marker rows which are offset to the right and the left, respectively, from the center of the lane and whose polarity arrays differ from each other and from that of the center marker row, as described previously.
Thus, the vehicle-onboard detection unit 3 can determine which of the marker rows the vehicle is currently traveling along on the basis of the polarities of the magnetic markers 1 detected in the past (step S6).
Finally, an offset quantity Or of the marker row from the center of the lane is added to the positional deviation Phc of the center position of the motor vehicle in the transverse direction relative to the magnetic marker 1 to thereby determine arithmetically a positional deviation Phr of the center position of the motor vehicle from the center of the lane in accordance with the undermentioned expression (3) (step S7):
Phr=Phc+Orxe2x80x83xe2x80x83(3)
In this manner, the final position of the motor vehicle in the transverse direction relative to the center of the lane (i.e., the deviation Phr) is arithmetically determined, whereon the processing routine shown in FIG. 9 comes to an end.
In the motor vehicle position recognizing system described above, it is noted that not only the characteristics of the magnetic markers 1 may change but also the types thereof may differ from one another. Besides, the heights of the magnetic sensors 2 mounted on the motor vehicle relative to the magnetic markers 1 may change in dependence on the conditions of the road surface (e.g. roughness, unevenness, snowfall, etc.). In that cases, a significant error may be brought about in the vehicle position determined arithmetically in particular when the magnetic sensor 2 whose output is destined for the arithmetic processing is changed over to another one.
In the following, by reference to FIGS. 11 to 13, description will be made in detail of the situation where error makes appearance the arithmetic determination of the vehicle position.
FIG. 11 is a view for illustrating an error xcex4Ph of the arithmetically determined value in the case where the height h of the magnetic sensor 2 relative to the road surface changes or increases by an amount xcex4h (hereinafter also referred to as the height change).
Further, FIG. 12 is a view for illustrating a relation between an actual position of the motor vehicle in the transverse direction (taken along the abscissa) and the arithmetically determined value Phr of the position of the motor vehicle in the transverse direction (taken along the ordinate) with the polarity of the height change xcex4h being changed to be positive and negative, respectively.
Furthermore, FIG. 13 is a view for illustrating a relation between an actual position of the motor vehicle in the transverse direction (taken along the abscissa) and the arithmetically determined value Phr of the position of the motor vehicle in the transverse direction (taken along the ordinate) when the magnetic sensor 2 whose output is destined for the arithmetic processing is changed over to another one. In the figure, characteristic of the arithmetically determined value of the vehicle position in the transverse direction is illustrated on the assumption that the polarity of the height change xcex4h is positive.
Referring to FIG. 11 and assuming, by way of example, that the height h of the magnetic sensor 2 relative to the magnetic marker 1 (i.e., relative to the road surface) becomes higher by a height change or increment xcex4h, the vehicle-onboard detection unit 3 will arithmetically determine the positional deviation Ph in the transverse direction on the presumption that the magnetic marker 1 exists at a position (indicated by a phantom line) higher than the actual position of the magnetic marker 1 (indicated by a solid line) by the height increment xcex4h.
Accordingly, the positional deviation Ph in the transverse direction determined arithmetically by the vehicle-onboard detection unit 3 assumes a value which is smaller than the actual positional deviation (=Ph+xcex4Ph) in the transverse direction by the error xcex4Ph.
More specifically, referring to FIG. 12, the slope of the characteristic straight line of the arithmetically determined value Phr of the vehicle position in the transverse direction relative to the vehicle position in the transverse direction (taken along the ordinate) becomes gentle when the height change xcex4h is of positive polarity (i.e., when xcex4h greater than 0) while it becomes steep when the height change xcex4h is of negative polarity (i.e., when xcex4h less than 0).
Consequently, when the position Phr of the motor vehicle in the transverse direction is arithmetically determined by processing the output of the magnetic sensor 2 which indicates the greatest absolute value among the peak vertical field intensity values Vv detected by a plurality of magnetic sensors 2, the arithmetic error becomes greatest when the above-mentioned magnetic sensor 2 is changed over to another one. As a result of this, jump or discontinuity makes appearance in the arithmetically determined value Phr of the vehicle position in the transverse direction, as illustrated at portions indicated as enclosed by broken line ellipses, respectively, in FIG. 13.
Parenthetically, although the above description has been made by reference to FIG. 13 on the assumption that the height change xcex4h is of positive polarity, it goes without saying that such discontinuity takes place similarly in the case where the height change xcex4h is of negative polarity.
As is apparent from the above description, when the magnetic sensor 2 whose output is destined to be processed for detecting the magnetic field intensity (the vehicle position in the running direction) is changed over from one to another in the course of traveling of the motor vehicle, discontinuity takes place in the arithmetically determined value Phr of the transverse vehicle position, bringing about remarkable error in the arithmetically determined value Phr of the transverse vehicle position, as a result of which the reliability of the automatic steering control and the lane departure alarming function is degraded or impaired, to a great disadvantage.
It will now be understood that the conventional motor vehicle position recognizing system suffers a problem that when the characteristic of the magnetic marker and the type thereof changes and/or when the magnetic sensor whose output is destined for the arithmetic processing is changed over at the time when the installation height of the magnetic sensor changes relative to the magnetic marker, there makes appearance noticeable error in the arithmetically determined value of the motor vehicle position.
In the light of the state of the art described above, it is an object of the present invention to provide a motor vehicle position recognizing system which can ensure high accuracy for the arithmetic processing for determining the vehicle position in the transverse direction by minimizing the error of the arithmetically determined value by providing a vehicle position correcting means for preventing or suppressing occurrence of the discontinuity even when the magnetic sensor whose output is destined for the arithmetic processing for determining the vehicle position is changed over.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a motor vehicle position recognizing system which includes a signal zone forming means laid on a road surface at least at one predetermined location for forming a signal zone of a predetermined signal strength, a plurality of signal strength detecting means mounted on a motor vehicle traveling on the road surface and disposed at a plurality of positions along a transverse direction substantially orthogonal to a traveling direction of the motor vehicle for detecting the signal strength of the signal zone, a vehicle position arithmetic means for arithmetically determining a position of the motor vehicle in a transverse direction as a vehicle position on the basis of the signal strength detected by at least one of the plural signal strength detecting means, a detecting means changeover means for changing over the signal strength detecting means whose output is used for an arithmetic operation for determining the vehicle position in dependence on maneuvering of the motor vehicle, and a vehicle position correcting means for correcting an arithmetically determined value of the position of the motor vehicle as determined arithmetically by the vehicle position arithmetic means in response to the changeover operation of the signal strength detecting means.
In a preferred mode for carrying out the invention, the vehicle position correcting means may be so designed as to correct the arithmetically determined value of the position of the motor vehicle in response to the changeover operation of the signal strength detecting means so that the arithmetically determined values of the positions of the motor vehicle as determined arithmetically before and after the changeover of the signal strength detecting means, respectively, assume substantially midpoints between arithmetically determined values based on the signal strengths detected by the individual signal strength detecting means before and after the changeover of the signal strength detecting means, respectively.
In another preferred mode for carrying out the invention, the vehicle position correcting means may be so designed as to correct the arithmetically determined value of the position of the motor vehicle so that coincidence is found among the vehicle positions determined on the basis of the detection signal strengths, respectively, when coincidence is detected among detection signal strengths derived from the outputs of the plural signal strength detecting means, respectively.
In yet another preferred mode for carrying out the invention, the signal zone forming means may be constituted by magnetic markers for generating magnetic fields of a predetermined intensity, and the plurality of signal strength detecting means may be constituted by a plurality of magnetic sensors for detecting the magnetic field intensities.
In still another preferred mode for carrying out the invention, the vehicle position correcting means may be so designed as to correct the arithmetically determined value of the position of the motor vehicle in response to the magnetic sensor changeover operation performed by the detecting means changeover means so that the arithmetically determined values of the positions of the motor vehicle as determined arithmetically before and after the changeover of the magnetic sensors, respectively, assume midpoints between arithmetically determined values based on the signal strengths detected by the magnetic sensors before and after the changeover of the magnetic sensors, respectively.
In a further preferred mode for carrying out the invention, the vehicle position correcting means may be so designed as to correct the arithmetically determined value of the position of the motor vehicle so that coincidence is found among the vehicle positions determined on the basis of the detected magnetic field intensities, respectively, when coincidence is detected among the detected magnetic field intensities derived from the outputs of the plurality of magnetic sensors, respectively.
In a yet further preferred mode for carrying out the invention, the plurality of magnetic sensors may be so designed as to detect magnetic field intensity in a vertical direction of the motor vehicle, and the vehicle position correcting means may be so designed as to correct the arithmetically determined value of the position of the motor vehicle so that coincidence is found among the vehicle positions determined on the basis of the detected magnetic field intensities detected by the plurality of magnetic sensors, respectively, when coincidence is detected among the vertical field intensities derived from the outputs of the plurality of magnetic sensors, respectively.
In a still further preferred mode for carrying out the invention, the motor vehicle position recognizing system further may further include a fault decision means for determining a fault of the magnetic sensor, wherein the fault decision means may be so designed as to determine that the magnetic sensor suffers a fault when the arithmetically determined values of the position of the motor vehicle differ at least by a predetermined value notwithstanding that coincidence is found among the detected vertical field magnetic intensities before and after changeover of the magnetic sensors.
In a mode for carrying out the invention, the plurality of magnetic sensors should preferably be so designed as to detect magnetic field intensity in a horizontal direction of the motor vehicle, and the vehicle position correcting means should preferably be so designed as to correct the arithmetically determined value of the position of the motor vehicle so that coincidence is found among the vehicle positions determined on the basis of the detected magnetic field intensities as detected by the plurality of magnetic sensors, respectively, when coincidence is detected among the horizontal field intensities derived from the outputs of the plurality of magnetic sensors, respectively.
In another mode for carrying out the invention, the motor vehicle position recognizing system should preferably include further a fault decision means for determining a fault of the magnetic sensor, wherein the fault decision means may be so designed as to determine that the magnetic sensor suffers a fault when the arithmetically determined values of the position of the motor vehicle differ at least by a predetermined value notwithstanding that coincidence is found among the detected horizontal field magnetic intensities before and after changeover of the magnetic sensors.
In yet another mode for carrying out the invention, the vehicle position correcting means should preferably be so designed that when the magnetic marker exists underneath one of the plural magnetic sensors, the vehicle position correcting means corrects the arithmetically determined value of the vehicle position such that the vehicle position determined on the basis of the detected field intensity derived from the output of the one magnetic sensor coincides with the vehicle position determined on the basis of the detected field intensity derived from the output of the magnetic sensor located adjacent to the above-mentioned one magnetic sensor.
In still another mode for carrying out the invention, the plurality of magnetic sensors may preferably be so designed as to detect magnetic field intensity in the horizontal direction of the motor vehicle, and the vehicle position correcting means may preferably be so designed that unless the horizontal field intensity is determined by one of the plurality of magnetic sensors, the vehicle position correcting means decides that the magnetic marker exists underneath the above-mentioned one magnetic sensor.
In a further mode for carrying out the invention, the motor vehicle position recognizing system should preferably be so arranged that when the position of the magnetic marker as arithmetically determined on the basis of the detected field intensity derived from the output of one of the plurality of magnetic sensors coincides with the position of the one magnetic marker, it is then decided that the magnetic marker exists underneath the above-mentioned one magnetic sensor.
In a yet further mode for carrying out the invention, the vehicle position arithmetic means may preferably be so designed that when the magnetic marker exists underneath the one magnetic sensor, the vehicle position arithmetic means arithmetically determines the correction factor so that the arithmetically determined value of the vehicle position as determined arithmetically on the basis of the field intensity detected by the magnetic sensor located adjacent to the above-mentioned one magnetic sensor coincides with a distance between the magnetic sensors.
In a still further mode for carrying out the invention, the motor vehicle position recognizing system may further include a fault decision means for making decision as to whether the magnetic sensor suffers a fault, wherein the fault decision means may be so designed as to decide that the magnetic sensor suffers a fault when the correction factor departs from a predetermined value range.
In another mode for carrying out the invention, the vehicle position correcting means should preferably be so designed as to correct the arithmetically determined value of the vehicle position such that the vehicle positions determined arithmetically on the basis of the detected field intensities derived from the outputs of the plurality of magnetic sensors, respectively, coincide with one another.
In yet another mode for carrying out the invention, the vehicle position arithmetic means should preferably be so designed as to arithmetically determined the correction factor such that a sum of absolute values of the arithmetically determined values of the vehicle positions determined on the basis of the detected field intensities derived, respectively, from the outputs of two of the plurality of magnetic sensors coincides with the distance between the above-mentioned magnetic sensors.
In still another mode for carrying out the invention, the motor vehicle position recognizing system may further include a fault decision means for making decision as to whether the magnetic sensor suffers a fault, and the fault decision means may be so designed as to decide that the magnetic sensor suffers a fault when the correction factor departs from a predetermined value range.
By virtue of the arrangements described above of the motor vehicle position recognizing system according to the present invention, the motor vehicle position recognizing system can ensure high accuracy for the arithmetic operation for determining the vehicle position in the transverse direction by minimizing the error otherwise involved in the arithmetic processing by providing the vehicle position correcting means for preventing or at least suppressing occurrence of the discontinuity even when the magnetic sensor whose output is destined for the arithmetic processing for determining the vehicle position is changed over to another one. Besides, it is possible to make decision as to whether or not the magnetic sensor suffers a fault, to another advantage.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.